1. Field of the Invention
The present invention relates to audio playback apparatuses and methods, and in particular relates to a multi-channel audio playback apparatus and method.
2. Description of the Related Art
Switching amplifiers, also named as class D amplifiers, are used as audio playback power amplifiers and have become more and more popular in portable devices due to their power efficiency. Moreover, switching amplifiers do not require heat sink devices to dissipate heat, thus, taking up less volume when used in portable devices.
FIG. 1 shows a schematic diagram of a conventional multi-channel audio playback apparatus. The multi-channel audio playback apparatus 100 comprises a serial-to-parallel data formatter 102, a switching amplifier 104, and loudspeakers 191 and 192.
The serial-to-parallel data formatter 102 receives multi-channel digital data 120 from a source (not shown) and separates the multi-channel digital data 120 in serial format into first channel digital data 121 and second channel digital data 122 in parallel format. As is well known in the art, the first channel digital data 121 and second channel digital data 122 can be left channel data and right channel data in a stereo audio system. Moreover, the serial-to-parallel data formatter 102 can separate the multi-channel digital data 120 into five channels which are left, right, center, left-back, right-back and subwoofer channels in a Dolby 5.1 system.
Taking a stereo audio system for example, the switching amplifier 104 further comprises a first digital-to-analog converter (DAC) 141, a second DAC 142, a reference signal generator 110, a first comparator 151, a second comparator 152, a first driver 161 and a second driver 162. The first DAC 141 and the second DAC 142 respectively convert the first channel digital data 121 and the second channel digital data 122 into first channel analog data 131 and second channel analog data 132. The reference signal generator 110 generates a reference signal 111 with a specific frequency and outputs the reference signal 111 to the first comparator 151 and the second comparator 152.
FIG. 2A illustrates the relationship between the first channel analog data 131 and the reference signal 111 of FIG. 1. The first comparator 151 receives the first channel analog data 131 from the first DAC 141 and the reference signal 111 from the reference signal generator 110 and compares the first channel analog data 131 with the reference signal 111 in order to generate the first pulse width modulation (PWM) signal 181. FIG. 2B illustrates the first PWM signal of FIG. 1. To explain in detail, when the first channel analog signal 131 is higher than the reference signal 111, the first PWM signal 181 is high (labeled as “1” in FIG. 2B). When the first channel analog signal 131 is lower than the reference signal 111, the first PWM signal 181 is low (labeled as “0” in FIG. 2B). FIG. 2C illustrates the relationship between the second channel analog data 132 and the reference signal 111 of Fig. 1 and FIG. 2D illustrates the second PWM signal 182 of FIG. 1. Accordingly, the second comparator 152 compares the second channel analog data 132 with the reference signal 111 in order to generate the second PWM signal 182. Then, the first driver 161 and the second driver 162 respectively use the first PWM signal 181 and the second PWM signal 182 to drive the first loudspeaker 191 and the second loudspeaker 192.
However, while the multi-channel audio playback apparatus 100 is playing sounds through the loudspeaker 191 and 192, severe radio frequency (RF) interference occurs. FIGS. 3A, 3B and 3C respectively shows the frequency spectrum of the first PWM signal 181, the second PWM signal 182 and combinations thereof of FIG. 1. The first PWM signal 181 in the frequency spectrum comprises a first channel audio frequency 312 corresponding to the first channel analog data 131 and a first carrier frequency 314 corresponding to the reference signal 111. Accordingly, the second PWM signal 182 in the frequency spectrum comprises a second channel audio frequency 322 corresponding to the second channel analog data 132 and a second carrier frequency 324 corresponding to the same reference signal 111, wherein the first carrier frequency 314 is the same as the second carrier frequency 324. However, while channel analog data 312 and 322 are being played as sounds from the loudspeakers, the carrier frequencies 314 or 324, in the range of 100 kHz˜400 kHz in most cases, contain non-ideal components in the PWM signals. Since most loudspeakers are made of magnetic materials, non-ideal components in the PWM signals radiate easily within the loudspeakers, thus affecting radio signals. In addition, with the same frequency (as shown in FIG. 3C), radio signals are further deteriorated when the amplitude of the second carrier frequency 324 is superposed onto the amplitude of the first carrier frequency 314. For example, the intensity of EMI caused by a 5.1 Dolby audio system is about 6 times higher than that caused by a mono-channel audio system.
As such, reducing RF interference of multi-channel audio playback apparatuses is desired.